Secondary battery and secondary battery manufacturing method

ABSTRACT

In a secondary battery, for providing a structure which can enable a welding operation even when a gap is formed between a current collecting plate and a winding assembly, recessed portions are formed in a positive current collecting plate. The recessed portions are disposed opposite to the winding assembly. A laser beam is irradiated to welding protrusions located between the recessed portions to melt the welding protrusions. Here, since the end surface of a positive electrode foil is uneven in height, the positive electrode foil does not contact the positive current collecting plate necessarily. The welding operation is performed by heating, melting, and dropping the welding protrusions by the use of a YAG laser under the welding condition of a laser power of 900 W and a welding speed 2 m/min.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation application of U.S. application Ser.No. 12/018,619, filed Jan. 23, 2008, the contents of which areincorporated herein by reference.

FIELD OF THE INVENTION

The present invention relates to a secondary battery and a secondarybattery manufacturing method, and more particularly, to a structure of abattery in which a winding assembly formed by winding a positiveelectrode and a negative electrode with a separator interposedtherebetween is received in a battery container along with anelectrolyte and in which electricity, which is charged in and dischargedfrom the winding assembly, can be taken from positive and negativeelectrode terminals to an outside.

DESCRIPTION OF RELATED ART

As lithium secondary batteries used in hybrid vehicles, batteries havinghigh power assisting ability (large current) have been required.Accordingly, in order to suppress the heat generation due to the largecurrent, it is necessary to reduce contact resistance inside thebatteries. In order to reduce the contact resistance inside thebatteries, it is necessary to increase the number of welding pointsbetween a current collecting plate and a winding assembly of thebatteries.

A conventional method of welding a current collecting plate and awinding assembly to each other by melting the current collecting platewhich is a flat plate and dropping the melted plate with the gravity isknown (see JP-A-2004-158394).

However, in the method disclosed in JP-A-2004-158394, since the currentcollecting plate is a flat plate, the length of the current collectingplate which is melted and dropped with the gravity is limited. InJP-A-2004-158394, it is described that the number of contact points isincreased by pushing and pressing the current collecting plate againstthe winding assembly. However, when the electrodes or metal foils of thewinding assembly are wound with a tension, and unevenness in height ofone of the wound metal foils at an end side of the winding assembly isgreat, the metal foil is not bent even by pushing and pressing thecurrent collecting plate against the metal foil. Accordingly, a gap isformed between the current collecting plate and the metal foil, therebynot satisfactorily increasing the number of contact points for welding.

BRIEF SUMMARY OF THE INVENTION

The invention is contrived to solve the above-mentioned problem. Anobject of the invention is to provide a secondary battery in which acurrent collecting plate is attached to a winding assembly by the use ofa joint design enabling a welding work even when a great gap existsbetween the current collecting plate and the winding assembly and amanufacturing method of the secondary battery.

In order to accomplish the above-mentioned object, according to anaspect of the invention, there is provided a secondary battery with astructure in which a winding assembly formed by winding a positiveelectrode and a negative electrode with a separator interposedtherebetween is received in a battery container along with anelectrolyte and in which electricity which is charged in and dischargedfrom the winding assembly can be taken from positive and negativeelectrode terminals to an outside, wherein a positive or negative metalfoil protrudes from at least one end of the winding assembly and acurrent collecting plate is welded to the edge of the protruding metalfoil. A plurality of recessed portions are formed in the currentcollecting plate before the welding and the current collecting plate isconnected to the metal foil by heating and melting convex portionsbetween the recessed portions. The welded portions of the currentcollecting plate after the welding form concave portions on the currentcollecting plate. That is, by heating the convex portions, the metal(welding metal) of the portions is melted and dropped with the gravityand can be connected to the metal foil. By solidifying the weldingmetal, the connection between the current collecting plate and the metalfoil is ensured. The welding portions of the current collecting plateare formed on the same side of the current collecting plate so as toconnect two outer peripheral edges of the winding assembly.

Further, the current collecting plate has at an outer periphery an outerperipheral bent portion which is bent toward the winding assembly. Ahole is formed at a center portion of the current collecting plate bypunching and an inner peripheral bent portion which is bent toward thewinding assembly is formed around the hole.

According to another aspect of the invention, there is provided amanufacturing method of a secondary battery with a structure in which awinding assembly formed by winding a positive electrode and a negativeelectrode with a separator interposed therebetween is received in abattery container along with an electrolyte, in which electricity whichis charged in and discharged from the winding assembly can be taken frompositive and negative electrode terminals to an outside, and in which acurrent collecting plate is welded to an edge of a positive or negativemetal foil protruding from at least an end side of the winding assembly.The secondary battery manufacturing method includes: preparing a currentcollecting plate having a plurality of recessed portions on a surfaceopposite to a surface in contact with the winding assembly; and weldingthe metal foil and the current collecting plate to each other, byheating convex portions between the recessed portions as weldingportions and melting down them with the gravity.

Other features of the invention will become apparent from the followingdescription and the accompanying drawings.

According to the invention, it is possible to obtain stable weldingquality even when there is a great gap between the current collectingplate and the winding assembly.

BRIEF DESCRIPTION OF SEVERAL VIEWS OF DRAWING

FIG. 1 is a development view illustrating a winding assembly of asecondary battery.

FIG. 2 is a perspective view illustrating a current collecting plate anda winding assembly of a secondary battery according to a firstembodiment of the invention.

FIG. 3 is a perspective view illustrating the current collecting plateand the winding assembly of the secondary battery according to the firstembodiment of the invention before they are welded to each other.

FIG. 4 is a cross-sectional view taken along line IV-IV in FIG. 3.

FIG. 5 is a cross-sectional view taken along line V-V in FIG. 3.

FIG. 6 is a perspective view illustrating the current collecting plateand the winding assembly of the secondary battery according to the firstembodiment of the invention after they are welded to each other.

FIG. 7 is a cross-sectional view taken along line VII-VII in FIG. 6.

FIG. 8 is a cross-sectional view taken along line VIII-VIII in FIG. 6.

FIG. 9 is a cross-sectional view illustrating an entire configuration ofa secondary battery.

FIG. 10 is a perspective view illustrating a current collecting plateand a winding assembly of a secondary battery according to a secondembodiment of the invention after they are welded to each other.

FIG. 11 is a perspective view illustrating a current collecting plateand a winding assembly of a secondary battery according to a thirdembodiment of the invention after they are welded to each other.

FIG. 12 is a perspective view illustrating a current collecting plateand a winding assembly of a secondary battery according to a fourthembodiment of the invention before they are welded to each other.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments according to the invention will be describedwith reference to the accompanying drawings. However, the embodimentsare only examples for carrying out the invention and are not intended tolimit the invention. In the drawings, the same elements are denoted bythe same reference numerals.

First Embodiment

FIG. 1 is a development view illustrating a known winding assembly. InFIG. 1, a winding assembly 5 is formed by winding plural members. In thewinding assembly 5, one member is a positive electrode foil 1 (forexample, an aluminum foil with a thickness of 10 μm) of which bothsurfaces are coated with a positive electrode material 2 and anothermember is a negative electrode foil 3 (for example, a copper foil) ofwhich both surfaces are coated with a negative electrode material 4. Thefoils are wound with a separator 10 interposed therebetween to form thewinding assembly 5. The winding assembly 5 is put into an electrolyte toserves as a secondary battery of being charged and discharged.

FIG. 2 is a perspective view illustrating a state of the embodimentaccording to the invention before a current collecting plate 6 isattached to the winding assembly 5. The positive electrode foil 1protrudes from the end side of the winding assembly 5 in a wound state.In order to serve as a secondary battery, it is necessary to connect(weld) the positive current collecting plate 6 to the positive electrodefoil 1 to transmit the electricity charged in and discharged from thewinding assembly 5.

FIG. 3 is a perspective view illustrating a state of the embodimentwhere the current collecting plate 6 is placed on the winding assembly 5before they are welded to each other. FIG. 4 is a cross-sectional viewtaken along line IV-IV in FIG. 3. The current collecting plate 6according to this embodiment has a shape obtained by cutting off bothend sides of a disc and includes two welding protrusions (convex lines)9 and four recessed portions (concave lines) 8 parallel to the line V-V(perpendicular to line IV-IV) on both sides of a center hole 30 of thecurrent collecting plate. The disposed positive current collecting plate6 is formed of an aluminum plate with a thickness of 1 mm and therecessed portions 8 are formed by a mechanical machining operation with,for example, a width of 1 mm and a depth of 0.5 mm. As shown in FIG. 4,the recessed portions 8 formed in the positive current collecting plate6 are disposed in the surface opposite to the surface contacting thewinding assembly 5. Then, by applying a laser beam to the weldingprotrusions 9 located between two recessed portions 8, the currentcollecting plate 6 is welded to the positive electrode foil 1. Forexample, the welding is performed by the use of a YAG laser under thewelding condition of a laser power of 900 W and a welding speed of 2m/min.

At this time, since the end face of the positive electrode foil 1 is noteven in height, the entire end of the positive electrode foil 1 comes incontact with the positive current collecting plate 6. If a laser beam isirradiated from the top surface of the flat current collecting platewithout the recessed portions 8, the melted metal does not droopsufficiently due to the surface tension of the melted metal (weldingmetal), and thus a portion of the positive electrode foil 1 may remainwithout contacting the welding metal. However, in this embodiment, sincethe recessed portions 8 are disposed on both sides of the weldingprotrusions 9, it is possible to reduce the influence of the surfacetension acting on the welding metal at the time of the application ofthe laser beam and the melted metal droops down enough to come incontact with the winding assembly 5 (see FIG. 7).

FIG. 5 is a cross-sectional view taken along line V-V of FIG. 3. Asshown in FIG. 5, the positive current collecting plate 6 includes aninner peripheral bent portion 7 and an outer peripheral bent portion 11which are formed by bending a flat plate toward the winding assembly 5.These bent portions are provided to prevent welding deformation. Thatis, if the positive current collecting plate 6 is a simple flat plate,an angular deformation or a warp deformation may be caused by thewelding. However, when the portions are bent at 90° with respect to thepositive current collecting plate 6 by means of the above bendingprocess, strength enough to endure the deformation at the time ofwelding the current collecting plate 6 is provided, thereby realizing awelding process without any welding deformation. The inner peripheralbent portion 7 is formed by extruding a flat plate which would be acurrent collecting plate, punching the center thereof, and then pressingthe resultant structure. The outer peripheral bent portion 11 is alsoformed by pressing the outer periphery of the extruded structure. It ispreferable to extrude and shape the current collecting plate 6 that thecurrent collecting plate 6 is symmetric.

FIG. 6 is a perspective view illustrating a welded state. As shown inFIG. 6, the positive current collecting plate 6 is welded at positionsof two welding metals 12 (positions corresponding to the weldingprotrusions 9). In the first embodiment, since the number of contactpoints between the positive current collecting plate 6 and the windingassembly 5 can be sufficiently secured from two welding positions, it isnot necessary to enlarge the size of the current collecting plate 6.Accordingly, it is possible to reduce the size of the current collectingplate 6 and thus to reduce the cost for the components and materials.According to this embodiment, since the welding operation is performedalong the welding positions on the same side, it is possible to reducethe number of welding operations. Therefore, it is not necessary torotate the secondary battery during the welding operation. As a result,it is possible to simplify the welding equipment.

FIG. 7 is a cross-sectional view taken along line VII-VII of FIG. 6. Asshown in FIG. 7, a laser beam is applied to the welding protrusions 9 tomelt metal of the corresponding portions and the welding metal 12 isdropped with the gravity. As described above, since the influence of thesurface tension on the welding metal 12 is reduced according to thefunction of the recessed portions 8, the welding metal 12 issufficiently melted and dropped and the positive electrode foil 1 andthe positive current collecting plate 6 are accordingly welded to eachother. That is, the unevenness in height of the end face of the positiveelectrode foil 1 is about 0.5 mm and thus the entire end face does nocome in contact with the positive current collecting plate 6. In thiscondition, according to this embodiment, the height by which the weldingmetal is melted and dropped can be made more than 0.5 mm. Accordingly,the positive electrode foil 1 and the positive current collecting plate6 can be welded to each other, even when a certain gap exists betweenthe positive electrode foil 1 and the positive current collecting plate6. In this embodiment, further, since the positive current collectingplate 6 is welded to the positive electrode foil 1 by melting anddropping, the welding operation can be performed in the state where thepositive electrode foil 1 is wound, and the positive electrode foil 1and the positive current collecting plate 6 can be welded to each othersubstantially at a right angle.

FIG. 8 is a cross-sectional view taken along line VIII-VIII of FIG. 6and shows a section of the center portion of the welding metal 12. Forexample, when the positive electrode foil 1 is wound in 12 turns, thewelding operation of one welding metal can weld at 24 positions byselecting the welding positions to connect the outer peripheral ends ofthe winding assembly 5. When the number of welding positions increases,the number of contact positions between the current collecting plate 6and the positive electrode foil 1 can increase, thereby reducing thecontact resistance between the positive electrode foil 1 and thepositive current collecting plate 6. This is advantageous particularlyfor a secondary battery with large current as described above. Accordingto this embodiment, as described above, since 24 contact positions canbe secured by the welding operation of one welding metal, it is notnecessary to perform a welding operation in a wide range so as toincrease the number of contact positions unlike the simple flat currentcollecting plate (JP-A-2004-158394), and it is possible to simplyperform the welding operation for a short time.

FIG. 9 is a cross-sectional view illustrating the entire structure of abattery. In FIG. 9, the winding assembly 5 in which the positiveelectrode foil 1 coated with the positive electrode material 2 and thenegative electrode foil 3 coated with the negative electrode material 4are wound with the separator 10 interposed therebetween is disposedaround a core 17 in a cell can 18, and the positive electrode foil 1 andthe negative electrode foil 3 are exposed on the respective end sides ofthe winding assembly 5. The positive electrode foil 1 and the positivecurrent collecting plate 6 are welded to each other by the method shownin FIGS. 2 to 8, and the positive electrode foil is electricallyconnected to an upper lid cap 15 through a path formed by the positivecurrent collecting plate 6, a lead 13, and an upper lid connecting plate16. That is, it is electrically connected to the outside. On the otherhand, the negative electrode foil 3 is electrically connected to thecell can 18 through a negative current collecting plate 20 and anegative electrode connecting plate 23. An electrolyte along with theelements is injected into the cell can 18 and the electricity is chargedin and discharged from the winding assembly 5. The invention relates tothe structures of connecting the positive electrode foil 1 to thepositive current collecting plate 6 and connecting the negativeelectrode foil 3 to the negative current collecting plate 20. Thenegative electrode party has the same structure as the positiveelectrode party but is different therefrom in material. The negativeelectrode foil 3 is formed of a copper foil with a thickness of 10 μmand the negative current collecting plate 20 is formed of a copperplate. The negative electrode foil 3 and the negative current collectingplate 20 are welded to each other by the use of the welding metal 21.

Second Embodiment

FIG. 10 is a perspective view illustrating a secondary battery accordingto a second embodiment of the invention after the welding operation isperformed. The secondary battery according to the second embodiment isequal to the secondary battery according to the first embodiment, exceptthat four welding metals 12 are used, and thus description thereof willbe omitted.

It is possible to obtain the same advantages as the first embodimentfrom a joint design according to the second embodiment.

Third Embodiment

FIG. 11 is a perspective view illustrating a secondary battery accordingto a third embodiment of the invention after the welding operation isperformed. The secondary battery according to the third embodiment isequal to the secondary battery according to the first embodiment, exceptthat two welding metals 12 are used, and thus description thereof willbe omitted. Since the number of welding positions is smaller than thoseof the secondary batteries according to the first and secondembodiments, the contact resistance cannot be lower than those of thefirst and second embodiments, but there is no problem in actual use.

It is possible to obtain the same advantages as the first embodimentfrom a joint design according to the third embodiment.

Fourth Embodiment

FIG. 12 is a cross-sectional view (corresponding to the cross-sectionalview of FIG. 4) illustrating a secondary battery according to a fourthembodiment of the invention before the welding operation is performed.In the fourth embodiment, the positive current collecting plate 6 ismade by a cold forging process (pressing process). Accordingly, therecessed portions 8 are formed in a circular-arc shape section and aweld reinforcement 22 may be formed outside them. The pressing processhas an advantage that it is lower in cost than the machining processaccording to the first embodiment.

It is possible to obtain the same advantages as the first embodimentfrom a joint design according to the fourth embodiment.

As described above, the embodiments provide a secondary battery with astructure in which a winding assembly formed by winding a positiveelectrode and a negative electrode with a separator interposedtherebetween is received in a battery container along with anelectrolyte and in which electricity which is charged in and dischargedfrom the winding assembly can be taken from positive and negativeelectrode terminals to the outside, wherein a positive or negative metalfoil protrudes from at least one end of the winding assembly and acurrent collecting plate is welded to the edge of the protruding metalfoil. A plurality of recessed portions are formed on the surface of thepre-welded current collecting plate opposite to the surface in contactwith the winding assembly, and the current collecting plate is connectedto the metal foil by melting and dropping convex portions between therecessed portions. In this way, it is possible to perform the weldingoperation even when a gap exists between the current collecting plateand the winding assembly. Since the thickness of the current collectingplate other than the welding portions can increase by providing therecessed portions, it is possible to reduce the welding deformation. Thecurrent collecting plate can be formed of an extruded material.

Since the welding portions of the current collecting plate are formed toconnect two outer peripheral edges of the winding assembly on the samesurface of the current collecting plate, it is possible to reduce thenumber of welding operations and to reduce the welding range on thecurrent collecting plate. Accordingly, since it is not necessary tochange the direction of the secondary battery during the weldingoperation, it is possible to simplify the welding process. Since thewelding range can be reduced, it is possible to reduce the size of thecurrent collecting plate and thus to reduce the cost for the components.Further, since the metal can be made to sufficiently droop down byheating the metal, it is possible to increase the number of connectionpoints to the metal foil and thus to reduce the contact resistancebetween the current collecting plate and the metal foil.

The current collecting plate has at the outer periphery an outerperipheral bent portion which is bent toward the winding assembly. Ahole is formed at the center portion of the current collecting plate bypunching process, and an inner peripheral bent portion which is benttoward the winding assembly is formed around the hole. By providing suchbent portions, it is possible to enhance the strength of the currentcollecting plate, thereby preventing the deformation of the currentcollecting plate due to the welding and thus performing the weldingoperation with high precision. As a result, it is possible to provide asecondary battery having positive and negative electrodes with stableshapes.

In addition, the embodiments provide a secondary battery manufacturingmethod, that is, a manufacturing method of a secondary battery with astructure in which a winding assembly formed by winding a positiveelectrode and a negative electrode with a separator interposedtherebetween is received in a battery container along with anelectrolyte, in which electricity which is charged in and dischargedfrom the winding assembly can be taken from positive and negativeelectrode terminals to the outside, and in which a current collectingplate is welded to an edge of a positive or negative metal foilprotruding from at least an end of the winding assembly. The secondarybattery manufacturing method includes: preparing a current collectingplate having a plurality of recessed portions on the surface opposite tothe surface in contact with the winding assembly; and welding the metalfoil and the current collecting plate to each other, by heating andmelting convex portions as welding portions between the recessedportions and by dropping them with the gravity. Accordingly, since thebad influence of the surface tension on the welding metal can beremoved, the metal can be made to sufficiently droop down by heating themetal, thereby increasing the number of connection points to the metalfoil. As a result, it is possible to reduce the contact resistancebetween the current collecting plate and the metal foil.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

1. A secondary battery with a structure in which a wound assembly formedby winding a positive electrode and a negative electrode with aseparator interposed therebetween is received in a battery containeralong with an electrolyte and in which electricity which is charged inand discharged from the wound assembly can be taken from positive andnegative electrode terminals to an outside, wherein a positive ornegative metal foil protrudes from at least an end of the woundassembly, and wherein said second battery has connected portions inwhich portions of a current collecting plate protruding in a directioncontacting with the wound assembly are connected to portions of themetal foil protruding in a direction contacting with the currentcollecting plate, an area to which said connected portion of the currentcollecting plate belongs is thicker than an area surrounding saidconnected portion of the current collecting plate, and an area outsideof said area surrounding the connected portion of the current collectingplate is thicker than the area surrounding the connected portion of thecurrent collecting plate.
 2. The second battery according to claim 1,wherein a surface of the area surrounding the connected portion of thecurrent collecting plate, opposite to a surface in contact with thewound assembly, is lower than a surface of the area outside of the areasurrounding the welding portion of the current collecting plate,opposite to a surface in contact with the wound assembly.
 3. The secondbattery according to claim 1, wherein the areas surrounding theconnected portion of the current collecting plate are an area which,before being connected, were a plurality of recessed areas provided on asurface of the current collecting plate opposite to a surface in contactwith the wound assembly, and wherein the area which the connectedportion of the current collecting plate belongs is an area which, beforebeing connected, was a convex portion formed between the plurality ofrecessed portions.
 4. The second battery according to claim 1, whereinthe connected portions of the current collecting plate are formed on thecurrent collecting plate on the same side.
 5. The second batteryaccording to claim 1, wherein the connected portions of the currentcollecting plate are formed to connect two outer peripheral ends of thewound assembly.
 6. The second battery according to claim 1, wherein thecurrent collecting plate has at the outer periphery an outer peripheralbent portion which is bent toward the wound assembly.
 7. The secondbattery according to claim 1, wherein a hole is formed at a centerportion of the current collecting plate by punching and an innerperipheral bent portion which is bent toward the wound assembly isformed around the hole.
 8. The second battery according to claim 1,wherein said connected portions are provided on both of the positive andnegative metal foils.
 9. The second battery according to claim 1,wherein said connected portions are connected by means of welding. 10.The second battery according to claim 1, wherein the portions of saidcurrent collecting plate protruding in the direction contacting withsaid wound assembly is formed by melting the current collecting plate.